Measuring instrument



March 23, 1948. J. E. @GATES 2,438,205

MEASURING INS'QRUMENT Filed Sept. 15, 1945` ,Y y2 Sheets-Sheet l IN VEN TOR.

March 23, 1948. J, E. CQATES l Mmsuamcr INSTRUMENT Filed Sept. 15, 1945 2 SheetS-Sheet 2 1N VEN TOR.

Patented Mar. 23, 1948 MEASURING INSTRUMENT J. Edwin Coates, Santa Monica, Calif., assignor to Douglas Aircraft Company,

Monica, Calif.

Inc., Santa Application September 15, 1945, Serial No. 616,515

1 11 Claims.

This invention relates to. measuring instru mentsand more particularly to a gauge for measuring the strain effects in elements or parts subjected to forces tending to produce strain therein.

The gauge of the present invention comprises in the broadest aspects thereof a body member to be mounted to a member subject to strain and carrying an elongate nlm-like mass of material forming a conductive path having a known resistance. The gauge is adapted to be connected to some electro-responsive means which will measure or evaluate the change in the resistance of the mass due to stresses imparted to the body member.

It has been heretofore proposed to mount to a body member a grid formed of fine resistance wire. body member by some suitable adhesive to the end that stresses exerted on the body member would be transmitted to the wire. These previously proposed instruments were difficult to fabricate for the fine resistance wire used to form the grid was exceedingly difficult to handle. This inherent defect complicated the fabrication of the instrument and also increased the manufacturing cost thereof.

The present invention obviates these difdculties for the grid of the gauge of the present invention is lformed by depositingr on the body or backing member a lm of conducting material of any size and configuration desired. The iilrn can be economically deposited on the body or backing member by any one of a number of methods now used to form or deposit iilms of various types.

One method of depositing the lm upon the body or backing member comprises the steps of forming upon the member a grid of the desired shape by means of a stencil formed with an opening of the shape of the grid desired and depositing through the opening onto the underlying body or backing member a metallic or other electrically conductive coating which might comprise copper or bronze powder carried by a suitable vehicle. As the grid thus formed is current conducting and will have either a known or determinable resistance the resulting gauge could be used as are the previously proposed instruments formed with a grid of ne resistance wire.

It is now preferred, however, to deposit upon the area coated by the base material a metallic layer of some suitable conducting material. 'I'his layer may be deposited upon the coated area by an electroplating process although, obviously, other conventional coating processes' could be used.

The resistance wire was cemented to the The base layer of the grid of the strain gauge of the present invention can also be formed by printing a grid of the desired shape upon the one face of the body or backing member by means of a conventional printing plate and a suitable metallic or current-conducting ink. It is also proposed to form the base layer of the grid by marking on the body or backing member with a stylus which when moved over the face of the body or backing member will mark the same with a current-conducting line of the desired conguration.

Although the grids formed bythe printing operation and the stylus will carry a cl'ir'rentl and could be used in some applications to measure strain, it is here again preferred to deposit upon the base layers thus formed a second layer through a process such as electroplating or other process in which the thickness of the deposited film can be controlled.

As the strain gauge of the present invention can be produced in mass byrneans of conventional operations, the fabrication of the same is comparatively simple. Thus the manufacturing costs of the gauge of the present invention will be considerably less than those of previously proposed instruments which required the handling of fine delicate resistance wire in the assembly thereof.

The body or backing member may comprise any sheet-like material desired such as paper, glass, or plastic, as the selection of the material of the body or backing member will be governed by the part undergoing test, and the temperatures attained by the same during the test.

Other features and advantages of the present invention will be apparent from the following description taken in connection with the accompanying drawing in which:

Figure 1 is a plan View of the instrument of the present invention:

Figure 2 is a sectional view on an enlarged scale taken along line 2--2 of Figure 1 and exaggerated for the purpose of more clearly illustrating the invention;

Figure 3 illustrates one step of one method of forming the gauge of the present invention;

Figure 4 is a view similar to Figure 3 but illustrating another method of forming the base layer of the grid of the instrument of the present invention; and

Figure 5 is another view similar to Figure 3 showing the initial step in a still further modified method of forming the grid of the instrument of the present invention.

The gauge or measuring instrument of the present invention, referring now to the drawing,

" 4and particularly Figure v1 ydesired shape in a stencil I1.

thereof, comprises a lbody or backing member I carrying an elongate film-like mass II vof conducting material. The film-like mass Il is so formed on the face of the body or backing member I0 that it forms a continuous electrical path of either known or determinable resistance. In the form of the invention illustrated, the film-like mass is formed in the shape of a grid having areas I2 at the opposite ends of the conducting path formed by the same y to which suitable leads I3 may be connected. In

the form of the invention now preferred the ends of the leads I3 are soldered to the areas I2 while the remaining portion of the leads overlying the body member I0 are connected to Ithe same .by some suitable adhesive material I4.

The body or backing member I0 may be formed of any suitable material in sheet form. The material of the Lbacking 4or body member I0 will be dictated to some extent by the particular use to which a gauge is to be put. Where the gauge is to be vused to measure strain of bodies not heated in use the body member l0 could be formedof some suitable paper. Where the gauge is to be used te measure strain in parts which are heated in use to temperatures above the charringA temperature of paper, the body member I0 could be formed of thin sheets of glassv or some plastic material having a softening temperature in ex'cess of the temperature which will .be attained by the test part.

A glass cloth formed of tightly woven filaments of relatively small diameters can beused as the body member wherethe gauge is to be used `to measure strain in parts normally heated, in use to relatively high temperatures. In this use, the

glass cloth can be attached to the part under.

going test by applying suilicient heat to the body member to cause the glass to soften and bond to the surface of the part.

In the illustrated embodiment of the present invention the film-like mass I I is formed or made up from two layers I5 and I5 which may be formed of the same or different materials. The

base layer I5 may be formed or deposited upon the body of the backing member I0 by any one of several different conventional methods.

This layer might be deposited upon the face of the body or backing member I0 by brushing or otherwise applying a suitable metallic or currentconducting material through an opening of the This method of applying the lm forming the layer I5 is illustrated in Figure 3. lThe opening I8 formed in the stencil I1 is such that as the material when brushed or otherwise applied to the same it will form an area of the desired configuration such as the shape of the grid shown by Figure 1.

The material applied through the opening of the stencil must be current-conducting and may comprise some metallic powder such as copper or bronze carried by a suitable vehicle which when dried would form a relatively hard material carrying the metallic powder therein. The powder should be present in the vehicle in such quantities that a conducting path will be established after the vehicle has dried and hardened.

Although .the grid formed by the base layer I5 Y is current conducting so that the gauge thus lted upon the area coated by an electro-plating the layer I5 although other conventional proc esses such as are now used to lay down or deposit metallic illms could be used.

To complete the gauge of the present invention after the base la'yer I5 has been. deposited the .body or backing member I0 can ybe placed in a suitable electroeplating bath in which the layer I5 would form the cathode. The body or backing member I0 would be allowed to remain in the bath for a time sumcient to build up a layer of the desired thickness. The instrument is then removed from the bath and after the same has dried the leads I3 are soldered to the areas I2 and adhesively engaged .throughout their length overlying the body member to the same.

The base layer I5 can `be formed by the method illustrated in Figure 4 wherein the grid II is formed on the body by means of a stylus I 9 movable over the backing or body member bymeans of control mechanism 20. The material iof the stylus I9 should be such as to mark the face of the body member with a layer suiiiciently current-conducting to permit the coated layer to be connected into the electro-plating circuit as the cathode so that the layer I6 can be deposited thereon. As in the earlier described forms of the invention, leads I3 are electrically connected to the areas I2 after the layer I5 has been deposited.

There is shown in Figure 5 another method by which the base coat I5 may be applied or deposited to the one face of the body member IU. In this method of forming the base layer I5 a printing plate 2l is formed with a raised portion 22 having a configuration such as that of the grid shown at II in Figure 1. The plate can be used in any conventional printing press for printing with some suitable metallic or other current-conducting ink a grid of the desired shape. It will -be understood that the ink used must contain suiiicient current-conducting particles so that the resulting grid will 4be currentconducting to the end that the layer i6 can be deposited through a coating process such as electro-plating. In some cases the bond between the current-conducting particles of the printed grid is not suillcient to withstand the tensile stresses set up in the grid in use. The deposition of the layer I6 strengthens the bond and renders the mass homogeneous.

After the layer I6 h'as been deposited upon the printed layer I5 the leads I3 can be connected to the areas I2 as in the earlier described form of the invention.

The various methods by which the gauge may beformed can be carried out by relatively unskilled personnel. It will be seen that the present invention provides methods for producing strain gauges in large quantities at relatively low manufacturing costs.

Although the now preferred embodiments of the present invention have been illustrated and described herein it is to be understood that the invention nee'd not be limited thereto for it is susceptible to changes in form and detail within the scope of the appended claims.

I claim: n

1. A strain gauge comprising: a body member adapted to be bonded to a member subject to strain; a grid carried by one face of said body member, said grid comprising a iilm of currentv conducting material bonded to said body member throughout its length so that strains induced in said body member will be transmitted to said vilm; and a layer of current-conducting material process. This process is now preferred to form V carried by said lm.

2. The method of forming a strain sauge which comprises: the steps oidepositing upon preselected portions of one face oi' a body member a band ofi current-conducting material: forming upon said band a second layer of current-conducting material; and thereafter attaching leads to the opposite ends of said band.

3. The method of forming a strain gauge which comprises: the steps of depositing upon preselected portions of the one face of a body member formed of non-conducting material a band of current-conducting material; forming upon said bandi a layer of current-conducting material the thickness of which is greater than the thickness ofzthe material of said band; and thereafter attaching leads to said current-conducting material.

4. The method of forming a strain gauge which comprises: the steps of depositing upon the one face of a body member of non-conducting material an elongate film of current-conducting material; placing said body member in an electro-plating bath as a cathode; and continuing the plating current until ya film of the desired thickness is deposited upon said firstnamed film. i

5. The method of forming a strain gauge which comprises: the steps of placing a stencil having an opening formed therein over a body member; applying through the opening of the stencil onto the one face of the body member a current-conducting material carried by a vehicle which will form a flexible lm when dry; removing said stencil and allowing said vehicle to harden and d`ry; thereafter depositing upon the area. coated a layer of conducting material; and attaching leads to said layer.

6. The method of forming strain gauges which comprises: the steps of printing with a currentconducting ink an area on the one face of a body member formed of insulating material; connecting the area printed as a cathode in an electro-plating solution; causing the current to flow through said bath until film of the desired thickness has been deposited upon the area printed; and thereafter electrically connecting leads to the film deposited.

7. The method of forming a measuring instrument of the type described comprising: the steps of marking a continuous band on a body member of insulating material with a marking member formed of current-conducting material; depositing upon the band a layer of currentconducting material of a thickness greater than the thickness of the material of the band to form a grid on the body member, the resistance of which will vary with changes in strain transmitted thereto; and electrically connecting leads to the opposite ends of the layer deposited on said band.

8. A strain gauge comprising: a body membery of insulating material adapted to have one surface thereof bonded to a member subject to strain; a grid carried by the other surface of said body member, said grid comprising a illm of nonmetallic current-conducting material bonded to :,saaaos said surface throughout its length so that the strains induced in said body member will be transmitted to said film; and a layer of metallic current-conducting material carried by and bonded to said film so that strains induced in said nlm will be transmitted to said metallic material the electrical resistance of which varies with changes of strain therein.

9. The method of forming a measuring instrument of the type described comprising: the steps of marking a continuous band on a body member of insulating material with a marking member formed of current-conducting material; forming a. layer of current-conducting material upon Said band by plating thereon a metallic material the resistance of which will vary in accordance with the changes in strain to which said body member is subjected; and thereafter electrically connecting leads to the opposite ends of said plated band.

10. The method of forming a measuring instrument of the type described comprising: forming a stencil having an opening therethrough of the desired conilguration; placing said stencil over the one surface of a flat body member of insulating material; depositing through the opening of said stencil a current-conducting material to form on said surface a grid of the configuration of said opening comprising a continuous electrical path, the resistance of which will vary lin accordance with changes in strain to which said body member is subjected; forming a layer of current-conducting material on said grid by plating on the material forming the same a metallic current-conducting material the electrical resistance of which will vary in accordance with changes of strain to which saidbody member is subjected; and electrically connecting leads to the opposite ends of the grid formed.

1l. The method of forming a measuring instrument of the type described which comprises: the steps of printing with an ink containing current-conducting particles a continuous elongate area on the one face of a body member formed of insulating material; depositing upon the area printed a layer of current-conducting material' the electrical resistance of which will vary in accordance with changes in strain `to which said body member is subjected; and thereafter electrically connecting leads to the opposite ends of the current-conducting area printed on said body member.

J. EDWIN COATES.

REFERENCES CITED The following references are of record in the ile of this patent:

UNITED STATES PATENTS Number Name Date 1,702,062 Sonkin Feb. 12, 1929 1,832,419 Pender Nov. 17, 1931 2,252,464 Kearnes et a1. Aug. 12, 1941 2,359,245 Ritzmann Sept. 26, 1944 2,373,676 Germeshausen 1 Apr. 17, 1945 2,380,514 Germeshausen II July 31, 1945 

